1. Field of the Invention
The invention relates to a heat sensitive recording material and in particular to a heat sensitive recording material excellent in color gradation reproduction.
2. Description of the Related Art
A heat sensitive recording method is advantageous in that (1) development is unnecessary, (2) when the support is a paper, its paper quality is similar to that of common paper, (3) handling is easy, (4) color optical density is high, (5) a recording apparatus is simple and inexpensive, and (6) there is no noise at the time of recording. Accordingly, use of heat sensitive recording is wide-spreading in fields such as facsimile and printer and label such as POS.
Under these circumstances, transparent heat sensitive recording materials capable of direct recording with a thermal head for coping with multiple colors or for projection of an image with an overhead projector (OHP), or for observing an image directly on a light table, have also been developed in recent years. There is increasing demand for image qualities with these heat sensitive recording materials.
As the sensitivity of the heat sensitive recording material is increased to cope with high-speed recording, an initial rise in color development tends to be steep in response to printing energy. As a result, image color gradation has been sacrificed.
To satisfy both sensitivity and image color gradation, a heat sensitive recording material having, on a support, a second coloring layer and a first coloring layer laminated thereon having the same coloring tone, wherein the first coloring layer forms color at lower temperatures than the second coloring layer, has been developed (see, for example, Japanese Patent Application Publication (JP-B) No. 6-30953). In this heat sensitive recording material, the coloring of the first coloring layer is prioritized in printing at low temperatures, while the coloring of the second coloring layer is utilized in printing at high temperatures.
A heat sensitive recording material excellent in color gradation reproduction, provided with a heat sensitive recording layer containing an electron-donating dye precursor-encapsulated microcapsule and an electron-accepting compound, has been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-216878).
It has also been proposed that a hollow particle-containing undercoat layer is arranged on a support to attain higher sensitivity of a heat sensitive recording material (see, for example, JP-A No. 5-573).